This application is based on an application No. H10-267896 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a display method for displaying a multilevel image based on a digital image input signal, and reduces the deterioration that occurs in image quality where there are insufficient display levels.
2. Description of the Prior Art
When displaying a multilevel image using a digital display apparatus such as a PDP (Plasma Display Panel), limitations on the number of colors that can be displayed by the display apparatus mean that smooth gradations between light and dark parts of an image cannot be displayed. As a result, brightness changes in steps, creating patterns composed of lines of equal brightness that spoil the displayed image.
One method for preventing such decreases in image quality is called error diffusion. In this method, limitations in the ability to display certain shades are compensated for by diffusion the difference (xe2x80x9cdisplay errorxe2x80x9d) between a value in the image signal that should be reproduced and the actual color used to display this value among the pixel values of the surrounding pixels. As one example, when an 8-bit (256-color) display is used to display an image expressed using 12 bits (4096 colors) per pixel, the lower four bits of each pixel value is set as the display error. As shown in FIG. 49, 7/16 of this display error is added to the pixel on the right, 3/16 of this display error is added to the pixel below and to the left, 5/16 of this display error is added to the pixel below, and 1/16 of this display error is added to the pixel below and to the right. The color used to display each pixel is then calculated according to the total of the input image signal that corresponds to the pixel and display errors that are added to this pixel due to the surrounding pixels.
This calculation is performed using the circuit shown in FIG. 50. Numeral 2001 in FIG. 50 represents the 12-bit input image signal, numeral 2002 represents the upper 8 bits of the output of the adder 2012, numeral 2003 represents the lower 4 bits of the output of the adder 2012, numerals 2004-2007 are multiplication units that multiply the display errors by the stipulated weightings, and numerals 2008-2011 are delay units for appropriately delaying the inputted display errors so that the display errors are diffused into the surrounding pixels. The adder 2012 adds the various values produced by the multiplication units 2004-2007 to the input image signal.
In this error diffusion process, the illustrated circuit calculates the sum of the original digital data (the input image signal) and the errors for four of the surrounding pixels that are inputted into the adder 2012 by the multiplication units 2004-2007. The upper 8 bits of this sum are outputted to the display apparatus and the lower 4 bits are diffused into the pixel values of the surrounding pixels.
In recent years, however, improvements in the performance of display apparatuses have led to increases in the frequency of image signals, so that the above calculation method is not fast enough to perform error diffusion processing for a modern display apparatus.
One potential solution to this problem would be to reduce the display frequency by using a shift register or the like to convert a serial input image signal into a multiphase signal so that digital data corresponding to a plurality of pixels that are adjacent in the scanning direction is input in parallel. Conventional error diffusion methods diffuse the display error of the pixel to the left of the target pixel into the pixel value of the target pixel. With such method, however, there is the drawback that several data cycles (a data cycle being the time taken to input the input image signal for one pixel into the circuits that perform the processing) would be required to determine the pixel values that should be displayed for all of the pixels in one set of multiphase data. This means that it would not be possible to output data as multiphase data (not that the concept of xe2x80x9cmultiphase dataxe2x80x9d is explained in detail in the Embodiments section of this specification).
The present invention was developed after an extension review of the problems stated above and has an object of providing a multilevel image display method that can perform error diffusion even when data is inputted as multiphase data.
The stated object is achieved by a multilevel image display method for a multilevel image display apparatus, the multilevel image display apparatus processing digital values corresponding to a plurality of pixels, which are adjacent in a scanning direction, in parallel as a data block and converting the digital values corresponding to each pixel in a data block into multilevel values that are used when displaying an image, the multilevel image display method including: an error calculation process for calculating a display error from a digital value that corresponds to a target pixel; and an error diffusion process for diffusing the display error calculated for the target pixel into digital values corresponding to pixels included in at least one data block that follows a data block including the target pixel.
With the stated construction, an image with what appear to be a large number of colors can be displayed due to error diffusion even when digital image data is inputted as multiphase data where pixel values for a plurality of pixels that are adjacent in the scanning direction are inputted in parallel. While the conventional method basically only diffuses the display error of the target pixel into a digital value of a pixel that is adjacent to the target pixel on the same scanning line, the present invention diffuses the display error of the target pixel into digital values of pixels in data blocks that are inputted after the data block that includes the target pixel. While conventional methods are incapable of diffusing errors for all pixels when data is inputted as multiphase data, the present invention is capable of such processing. In short, amended data can be outputted with the same number of phases as the input multiphase data. The diffusion of errors into digital values corresponding to pixels in data blocks that are inputted after the data block including the target pixel can be performed according to the techniques described below.
Here, the error diffusion process may diffuse the calculated display error into digital values corresponding to pixels that lie on scanning lines which are below a scanning line that includes the target pixel.
With the above technique, more time is available for the calculations for error diffusion that needed to be performed within one data cycle in conventional error diffusion methods. This means that the error diffusion processing can be performed by relatively low-speed circuitry.
Here, the error diffusion process may diffuse the calculated display error into digital values of pixels in data blocks that come after the data block including the target pixel, said pixels having a same position (hereafter, xe2x80x9cphasexe2x80x9d) in a data block as the target pixel.
With the above technique, error diffusion processing is performed separately for the data in each phase of the multiphase input signal, so that the construction of the circuitry can be simplified.
Here, when error diffusion process diffuses the calculated display error into digital values of pixels on a same scanning line as the target pixel, the display error may be diffused into pixels that have a same phase within a data block as the target pixel, and when the error diffusion process diffuses the calculated display error into digital values of pixels on a lower scanning line, the display error may be diffused into pixels that are adjacent to the target pixel.
If display errors are only diffused into digital values of pixels with the same phase, the pixels that are affected by a target pixel will be spatially separated from the target pixel, which can mean that there will be little correlation between such pixels and that the positive effects on image quality due to the diffusion of errors will be weakened. However, with the above technique, display errors are also diffused into the digital values of neighboring pixels that have a high correlation with the target pixel, so that the positive effects on image quality due to the diffusion of errors can be maintained. Also, by diffusing a display error into a digital value of a pixel that is spatially separated from the target pixel but is present on the same line as the target pixel, the effects of the display error can be spread out over a wider area, which means that an image of a similar high standard to conventional error diffusion methods can be obtained.
Here, when a digital value corresponding to a pixel that is adjacent to the target pixel on a same scanning line will be processed at least one data cycle after the digital value of the target pixel, the error diffusion process may diffuse the calculated display error into the digital value corresponding to the pixel that is adjacent to the target pixel on the same scanning line, and in all other cases, the error diffusion process may diffuse the calculated display error into other pixels whose digital values will be processed at least one data cycle after the digital value of the target pixel.
With the above technique, display errors are diffused in a wide, fan-shaped pattern around the target pixel. By doing so, pixel values can be averaged over a wide area, so that smoother color gradations can be produced. By additionally diffusion the display error in the scanning direction, the display error of the target pixel can be diffused in a neighboring pixel that can have the highest correlation with the target pixel, which means that an image of a similar high standard to conventional error diffusion methods can be obtained.
As described above, the display error of a target pixel is diffused into digital values of pixels in data blocks that come after the data block including the target pixel.
Here, the display errors calculated by the error calculation process may include positive and negative values.
The above technique produces an image with a higher quality than techniques that only use positive values as display errors.
The error diffusion process may select one out of a plurality of patterns that are prepared in advance, each pattern diffusing the display error calculated for a target pixel into digital values of other pixels.
Here, the error diffusion process may use four patterns, the four patterns including: two patterns which diffuse the calculated display error into digital values corresponding to four consecutive pixels that are near the target pixel on a scanning line that is immediately below a scanning line including the target pixel, one of said two patterns diffusing the calculated display error into the digital values with weightings that are in a small, large, small, large arrangement along a scanning direction and another of said two patterns diffusing the calculated display error into digital values with weightings that are in a large, small, large, small arrangement along the scanning direction; and two patterns which diffuse the calculated display error into (1) a digital value of one pixel that is adjacent to the target pixel on a same scanning line as the target pixel, and (2) digital values of three consecutive pixels that are near the target pixel on a scanning line that is immediately below the scanning line including the target pixel, one of said two patterns diffusing the calculated display error into the four digital values with weightings that are in a small, large, large, small arrangement for a given order of the four digital values and another of said two patterns diffusing the calculated display error into the four digital values with weightings that are in a large, small, small, large arrangement that is opposite to the given order.
By suitably combining the plurality of error diffusion patterns, the above technique can perform a variety of processes that prevent deterioration in image quality due to a regular distribution of bright pixels across images, and so can output high-quality images. Note that it is preferable for the total weighting of display errors that are diffused into heavily affected pixels to be 1.5-3 times the total weighting of display errors diffused into light affected pixels. This distribution is used since a certain difference needs to be maintained between the total display error diffused into heavily affected and lightly affected pixels to prevent the creation of consecutive bright pixels. However, if this difference is too big, heavily affected pixels will definitely become bright, so that an undesirable pattern that reflects the arrangement of the error diffusion patterns will be observed in the display image.
From the above perspective, it is preferable for the two patterns which diffuse the calculated display error into digital values corresponding to four consecutive pixels that are near the target pixel on a scanning line that is immediately below the scanning line including the target pixel to respectively use (1) 3/16, 6/16, 2/16, 5/16 and (2) 6/16, 2/16, 6/16, 2/16 as the arrangements of weightings, and for the other two patterns to be (i) a pattern that diffuses 7/16 of the calculated display error into the digital value of the pixel on the same scanning line as the target pixel and 6/16, 2/16, and 1/16 of the calculated display error respectively into the digital values of the pixels on the scanning line that is immediately below the scanning line including the target pixel and (ii) a pattern that diffuses 1/16 of the calculated display error into the digital value of the pixel on the same scanning line as the target pixel and 2/16, 7/16, and 6/16 of the calculated display error respectively into the digital values of the pixels on the scanning line that is immediately below the scanning line including the target pixel.
Here, the error diffusion process may use two patterns which diffuse the calculated display error into digital values corresponding to four consecutive pixels that are near the target pixel on a scanning line that is immediately below a scanning line including the target pixel, one of said two patterns diffusing the calculated display error into the pixels with weightings that are in a small, large, small, large arrangement along a scanning direction and another of said two patterns diffusing the calculated display error into the pixels with weightings that are in a large, small, large, small arrangement along the scanning direction.
By suitably combining the plurality of error diffusion patterns, the above technique can perform a variety of processes that prevent deterioration in image quality due to a regular distribution of bright pixels across images, and so can output high-quality images. Note that it is preferable for the total weighting of display errors that are diffused into heavily affected pixels to be 1.5-3 times the total weighting of display errors diffused into lightly affected pixels. This distribution is used since a certain difference needs to be maintained between the total display error diffused into heavily affected and lightly affected pixels to prevent the creation of consecutive bright pixels. However, if this difference is too big, heavily affected pixels will definitely become bright, so that an undesirable pattern that reflects the arrangement of the error diffusion patterns will be observed in the display image.
From the above perspective, it is preferable for the two patterns to respectively use (1) 3/16, 6/16, 2/16, 5/16 and (2) 6/16, 2/16, 6/16, 2/16 as the arrangements of weightings along the scanning direction.
Here, the error diffusion process may use two patterns, a first of the two patterns diffusing the calculated display error into digital values of three pixels composed of a first pixel at a position that is on a same scanning line as the target pixel but is separated from the target pixel by several pixels in a first direction, a second pixel that is adjacent to the target pixel and lies on a scanning line that is immediately below the scanning line including the target pixel, and a third pixel that lies on a same scanning line as the second pixel and is separated from the target pixel by several pixels in the first direction, and a second of the two patterns diffusing the calculated display error into digital values of three pixels composed of a fourth pixel at a position that is on a same scanning line as the target pixel but is separated from the target pixel by several pixels in the first direction, a fifth pixel that is adjacent to the target pixel and lies on a scanning line that is immediately below the scanning line including the target pixel, and a sixth pixel that lies on a same scanning line as the fifth pixel and is separated from the target pixel by several pixels in a different direction from the first direction.
By suitable combining the plurality of error diffusion patterns, the above technique can prevent deterioration in image quality due to a regular distribution of bright pixels across images. Since display errors are diffused into digital values for three pixels with the same phase as the target pixel only, the number of multiplication units can be decreased and separate error diffusion processing can be performed for each phase. This simplifies the circuit construction. Note that it is preferable for the distribution (weightings) of the display errors diffused from the target pixel in each pattern to be similar. This distribution is used since a certain difference needs to be maintained between the total display error diffused into heavily affected and lightly affected pixels to prevent the creation of consecutive bright pixels. However, if this difference is too big, heavily affected pixels will definitely become bright, so that an undesirable pattern that reflects the arrangement of the error diffusion patterns will be observed in the display image.
From the above perspective, it is preferable for the first pattern to diffuse the calculate display error with a weighting of 5/16 into the digital value of the first pixel, with a weighting of 7/16 into the digital value of the second pixel, and with a weighting of 4/16 into the digital value of the third pixel, and for the second pattern to diffuse the calculated display error with a weighting of 7/16 into the digital value of the fourth pixel, with a weighting of 5/16 into the digital value of the fifth pixel, and with a weighting of 4/16 into the digital value of the sixth pixel.
Here, the error diffusion process may use two patterns, both patterns diffusing the calculated display error into digital values of four pixels composed of a first pixel at a position that is on a same scanning line as the target pixel but is separated from the target pixel by several pixels in a first direction, a second pixel that is adjacent to the target pixel and lies on a scanning line that is immediately below the scanning line including the target pixel, a third pixel that lies on a same scanning line as the second pixel and is separated from the target pixel by several pixels in the first direction, and a fourth pixel that lies on a same scanning line as the second pixel and is separated from the target pixel by several pixels in a second direction that differs from the first direction, the two patterns including different weightings for diffusing the calculated display errors into the digital data of the four pixels.
By suitably combining the plurality of error diffusion patterns, the above technique can prevent deterioration in image quality due to a regular distribution of bright pixels across images. Since display errors are diffused into digital values for three pixels with the same phase as the target pixel only, the number of multiplication units can be decreased and separate error diffusion processing can be performed for each phase. This simplifies the circuit construction.
Note that it is preferable to diffuse around 5/16 -7/16 of the display error into the digital values of a pixel on the same scanning line as the target pixel at a distance of several pixels from the target pixel in the first direction, around 1/16-3/16 of the display error into the digital values of a pixel on the next scanning line as the target pixel at a distance of several pixels from the target pixel in the first direction, and the rest of the display error into the digital values of (1) a pixel on the next line that is adjacent to the target pixel and (2) a pixel on the next line at a distance of several pixels from the target pixel in the second direction. This distribution is used since to prevent the creation of consecutive bright pixels, a certain difference needs to be maintained between the total display error diffused into heavily affected and lightly affected pixels. However, if this difference is too big, heavily affected pixels will definitely become bright, so that an undesirable pattern that reflects the arrangement of the error diffusion patterns will be observed in the display image.
From the above perspective, it is preferable for a first of the two patterns to diffuse 7/16 of the calculated display error into the digital value of the first pixel, 1/16 of the calculated display error into the digital value of the third pixel, 5/16 of the calculated display error into the digital value of the second pixel, and 3/16 of the calculated display error into the digital value of the fourth pixel, and for a second of the two patterns to diffuse 1/16 of the calculated display error into the digital value of the first pixel, 7/16 of the calculated display error into the digital value of the third pixel, 3/16 of the calculated display error into the digital value of the second pixel, and 5/16 of the calculated display error into the digital value of the fourth pixel.
Here, the error diffusion process may use two patterns, both patterns diffusing the calculated display error into digital values of four pixels composed of a first pixel at a position that is on a same scanning line as the target pixel but is separated from the target pixel by several pixels in a first direction, and three consecutive pixels that are near the target pixel and lie on a scanning line that is immediately below the scanning line including the target pixel, the two patterns including different weightings for diffusing the calculated display errors into the four pixels.
By suitably combining the plurality of error diffusion patterns, the above technique can prevent deterioration in image quality due to a regular distribution of bright pixels across images. By diffusing the display error in the scanning direction as in conventional error diffusion methods, the display error of the target pixel can be diffused in a neighboring pixel that can have high correlation with the target pixel. Diffusing the display error into a pixel on the same scanning line also means that display errors are diffused over a wider area, which means that an image of a similar high standard to conventional error diffusion methods can be obtained.
Note that it is preferable to diffuse around 5/16-8/16 of the display error into the digital values of a pixel on the same scanning line as the target pixel at a distance of several pixels from the target pixel in the first direction, and the rest of the display error roughly equally into the digital values of the other two pixels. This distribution is used since to prevent the creation of consecutive bright pixels, a certain different needs to be maintained between the total display error diffused into heavily affected and lightly affected pixels. However, if this difference is too big, heavily affected pixels will definitely become bright, so that an undesirable pattern that reflects the arrangement of the error diffusion patterns will be observed in the display image.
From the above perspective, it is preferable for a first of the two patterns to diffuse 8/16 of the calculated display error into the digital value of the pixel that is on the same scanning line as the target pixel and 2/16, 5/16 and 1/16 of the calculated display error in order along a scanning direction respectively into the digital values of the three consecutive pixels that lie on a scanning line that is immediately below the scanning line including the target pixel, and for a second of the two patterns to diffuse 2/16 of the calculated display error into the digital value of the pixel that is on the same scanning line as the target pixel and 7/16, 1/16 and 6/16 of the calculated display error in order along the scanning direction respectively into the digital values of the three consecutive pixels that lie on a scanning line that is immediately below the scanning line including the target pixel.
Note that when the display error of a target pixel is diffused into the digital value of a pixel on a scanning line that is directly below the scanning line including the target pixel, it is assumed that the number of phases in the multiphase digital data is no greater than the number of pixels in one scanning line.
When diffusing the display error into the digital values of nearby pixels, a plurality of patterns may be interchanged along the scanning direction according to a cyclical arrangement for a number of pixels, so that a same pattern is not used for pixels that are adjacent in the scanning direction.
With the above technique, it is possible to avoid the cyclical appearance of bright pixels in the scanning direction which would lower image quality and would occur if the patterns were not interchanged.
The interchanging of patterns along the scanning direction according to a cyclical arrangement may be such that the total weighting of display errors added to adjacent pixels in the scanning direction alternates between large and small values.
With the above technique, it is possible to avoid the cyclical appearance of consecutive bright or dark pixels in the scanning direction which would lower image quality.
When diffusing the display error into the digital values of nearby pixels, a plurality of patterns may be interchanged for each scanning line, so that a same pattern is not used for pixels that are adjacent in the scanning direction or for pixels that are adjacent in a direction perpendicular to the scanning direction.
With the above technique, it is possible to avoid the cyclical appearance of bright pixels in the direction perpendicular to the scanning direction which would lower image quality and would occur if the patterns were not interchanged for each scanning line.
The interchanging of patterns for each scanning line may be such that the total weighting of display errors added to adjacent pixels in the direction perpendicular to the scanning direction alternates between large and small values.
With the above technique, it is possible to avoid the cyclical appearance of consecutive bright or dark pixels in the direction perpendicular to the scanning direction which would lower image quality.
When diffusing the display error into nearby pixels, the patterns may be changed for each TV field, so that a same pattern is not used for a same target pixel in consecutive TV fields.
With the above technique, it is possible to avoid the appearance of fixed bright and dark areas on the screen which would lower image quality and would occur if the patterns were not interchanged for each scanning line.
The interchanging of patterns for each field may be performed so that the total weighting of display errors added to a same pixel alternates between large and small values.
The above technique averages the display time of light pixels and dark pixels and so enables intermediate color to be displayed.
The interchanging of patterns for consecutive scanning lines and for a same scanning line in different TV fields may be random.
By doing so, it is possible to prevent the occurrence of undesirable patterns of regularly appearing bright or dark pixels that may be observed in a moving image.
A motion detection unit may also be used and the switching of patterns may be controlled in accordance with whether motion has been detected by the motion detection unit.
With the above technique, optimal switching of patterns can be achieved for both moving and still images.
In parts of an input image where the motion detection unit detects no motion, the patterns may be cyclically interchanged so that a same pattern is not used for pixels that are adjacent in the scanning direction, for pixels that are adjacent in a direction perpendicular to the scanning direction, and for a same pixel in different TV fields.
With the above technique, bright pixels and dark pixels can be averaged both spatially and over time, so that smooth color gradations can be displayed, with the additional suppression of noise which would occur if the patterns were interchanged at random.
In parts of an input image where the motion detection unit detects motion, the patterns may be cyclically interchanged so that a same pattern is not used for pixels that are adjacent in the scanning direction, and patterns may be randomly interchanged for pixels that are adjacent in a direction perpendicular to the scanning direction and for a same pixel in different TV fields.
If patterns are interchanged in moving parts of an image, checkerboard patterns may be observed as the viewer""s eyes follow the moving image. However, if patterns are randomly interchanged for pixels that are adjacent in a direction perpendicular to the scanning direction and for a same pixel in different TV fields, such phenomenon can be avoided.